My experience with used bikes is the issues are something you would never notice riding it. Bolts over torqued and you find out when you need to adjust something, a bolt is stripped. Little things like that. I don't think there is an effective way to diagnose those though.

@ chaddb, I just bought a used bike... if what you mentioned does rear its ugly head on me... what can I do to fix it? Both the worst and best case scenarios please. I bought it off eBay under the presumption that it had only had about 3-5 rides, nothing big, no falls, small or big. However, there is always that "gray" area in buying used... but the price was in my ballpark (a little on the too good to be true side though). It was still quite expensive, so I am in no position to drop a bunch into fixing it.

Nervous... things to look for? Any specifics to really worry about (safety and longevity issues)?